1. Field of the Invention
The present invention relates to a joined (cemented) prism assembly and also to a method and an apparatus for manufacturing the same.
2. Description of the Related Art
A dichroic prism made of a joined prism assembly (that is, a rectangular parallelepiped prism) is used as a component of a projection display device.
The projection display device is used, for example, as a liquid crystal color projector wherein R (red), G (green) and B (blue) illuminating light rays are applied to three liquid crystal panels which give R, G and B color separation information, respectively, to obtain separate R, G and B images which are then combined and projected in the form of an enlarged color image through a projection lens.
FIG. 7 is a conceptual or schematic view of a device which employs a dichroic prism and dichroic mirrors for separation and combination (synthesis) of light rays, as an example of a conventional liquid crystal color projector of the type described above. Liquid crystal panels 32, 33 and 34 for R, G and B are disposed in opposing relation to three surfaces (defined as incident surfaces), respectively, of a dichroic prism 31 which is in the shape of a rectangular parallelepiped, and a projection lens 35 is disposed so as to face the remaining one surface (defined as an emergent surface E). A R (Red) reflecting dichroic mirror 32a, a G (green) reflecting dichroic mirror 33a and a B (blue) reflecting dichroic mirror 34a are disposed on the respective incidence sides of the liquid crystal panels 32, 33 and 34 at a 45.degree. angle with respect to the associated optical axes. Illuminating light rays are made incident on these dichroic mirrors 32, 33 and 34. The illuminating light rays are obtained by passing light from a white light source 36 through a collimating condenser lens 37, thereby producing collimated (parallel) rays of light. The parallel light rays are applied successively to the B-reflecting dichroic mirror 34a, a total reflecting mirror 38, the G-reflecting dichroic mirror 33a, a total reflecting mirror 39 and the R-reflecting dichroic mirror 32a in this order.
The dichroic prism 31 is provided therein with a B (blue) reflecting film 31a and a R (red) reflecting film 31b which are at 45.degree. to the corresponding optical axes and which are perpendicular to each other. Both the reflecting films 31a and 31b transmit G (green) light therethrough.
In the liquid crystal color projector, therefore, when the liquid crystal panels 32, 33 and 34 are irradiated with parallel light rays produced by a combination of the white light source 36 and the collimating lens 37 while these panels 32, 33 and 34 are being switched on the basis of the corresponding color information, red, green and blue light rays are respectively transmitted through the liquid crystal panels 32, 33 and 34 and enter the dichroic prism 31. The incident light rays are combined (superposed) through the B- and R-reflecting films 31a and 31b provided on the prism 31, and the combined picture image emerges from the emergence surface E of the prism 31 and is projected in the form of an enlarged image on a screen 40 through the projection lens 35.
The dichroic prism 31 used in the projection display device consists of four right-angle prisms 31A, each having an isosceles right triangle cross sectional shape. The right angle prisms 31A are joined to each other so that the adjacent sides of the triangles which define the right angle apexes are connected to each other. In the dichroic prism, a precise perpendicularity (squareness) of the joined surfaces of the four prisms and a strict linearity (i.e., flatness of the B-reflecting film 31a and the R-reflecting film 31b) are required, since otherwise a deviation of a projected and superposed image occurs.
However, to realize high perpendicularity and linearity of the joined surfaces of the four right prisms 31A which constitute a dichroic prism 31, it is necessary to join the four right prisms 31A with the help of a microscope through which an operator observes. This is, however, a very troublesome and complicated operation; moreover, it is not efficient.